(1) Field of the Invention
This invention relates to an optical transmission apparatus and, more particularly, to an optical transmission apparatus having the function of monitoring optical performance.
(2) Description of the Related Art
With an increase in the amount of information on communication networks, a growing number of optical transmission apparatuses using optical fibers have been supplied. In addition, there is a demand for high-speed large-capacity optical transmission. High-speed optical transmission is required, while importance has been attached to an optical performance monitoring (PM) function for accurately informing users of light quality.
FIG. 30 is a view showing a conventional optical transmission apparatus having a performance monitoring function. An optical transmission apparatus 100 which transmits light of one wavelength comprises a laser section 101, an O/E section 102, an amplifier section 103, an A/D conversion section 104, and a PM display section 105.
The laser section 101 outputs light of one wavelength. The O/E section 102 makes an optical/electrical conversion to output, for example, a laser bias current (LBC), being a bias current which flows to a laser, as a voltage value. This voltage value is directly proportional to the value of a bias current.
Output voltages vary according to the type of a laser, so the amplifier section 103 amplifies them to a constant value (between 0 and 4 V.) The A/D conversion section 104 makes an A/D conversion on a signal output from the amplifier section 103 to convert information indicative of a voltage value to an 8-bit digital signal.
The PM display section 105 converts an 8-bit digital signal to a display value by the use of a conversion table 105a and displays it on a user's terminal 2.
FIG. 31 is a view showing the conversion table 105a. The conversion table 105a gives display values corresponding to 256 pieces of 8-bit information. In this example, the pieces of 8-bit information from 00h to FFh are divided into twenty-one groups and one of the twenty-one values from 0.0 to 2.0 which are given in increments of 0.1 is assigned to each group. Therefore, if an 8-bit information value is one of, for example, 00h through 09h, then the value 0.0 will be displayed to a user.
Moreover, an adjustment is made individually at production time with initial variations in output voltage due to, for example, a difference in laser lot taken into consideration so that voltage output from the amplifier section 103 will be 2 V. As a result, a display value will be 1.0 (=reference value) at initial operation time at the stage of shipping a product. When light quality changes due to, for example, secular degradation or variations in temperature (when a bias current changes in the case of an LBC), a display value to a user will change by the zero point one. A user can manage light quality by monitoring a display value indicated in the conversion table 105a. 
On the other hand, in recent years narrow band (NB) tunable optical transmission apparatuses have been developed. With an NB tunable optical transmission apparatus, a plurality of (n) one-wavelength waves can be used and one of them can be selected tunably. As a result, a user can reduce the number of optical transmission apparatuses he/she should possess, compared with a case where he/she uses optical transmission apparatuses each of which outputs only one one-wavelength wave. In addition, he/she need only ensure another NB tunable optical transmission apparatus as a backup. This is efficient.
By exerting conventional optical performance monitoring control over the above NB tunable optical transmission apparatus, however, display values vary according to wavelengths even if there is no change in light quality.
FIG. 32 is a view showing LBCs corresponding to a plurality of wavelengths. Vertical and horizontal axes in FIG. 32 indicate optical power transmitted (OPT) and an LBC respectively.
When the value of OPT, being light output power, is a (normal value), the values of LBCs for light of the wavelengths λ1, λ2, λ3, and λ4 are b1, b2, b3, and b4 respectively. That is to say, if there is a difference in wavelength, the values of LBCs differ from one another even in the case of light output power being the same. It is assumed that the values b1, b2, b3, and b4 correspond to the display values 0.4, 0.6, 0.8, and 1.0 respectively.
Therefore, the reference value 1.0 is displayed only when light of the wavelength λ4 is used. Even if the values of the output power of light of the wavelengths λ1, λ2, λ3, and λ4 are the same and normal, values other than 1.0 will be displayed for light of the wavelengths λ1, λ2, and λ3.
It is assumed that a wavelength used in an NB tunable optical transmission apparatus is set at first to λ4 and that the apparatus is operated. Then a display value will be 1.0 (=reference value). If the wavelength is changed afterward from λ4 to λ1, a display value will be 0.4. That is to say, degradation in light quality will be displayed, but in reality a light output level is normal.
As stated above, even if light output power is normal, reference values vary according to wavelengths. Therefore, when an abnormality has occurred in an NB tunable optical transmission apparatus, an alarm are not generated.
For example, setting is made so that an alarm will be generated when a display value increases to 1.5 (one and a half times the reference value 1.0). If the wavelength λ4 is used, then an alarm will be generated when a display value increases to 1.5. However, if the wavelength λ1 is used, an alarm will not be generated because one and a half times the reference value is zero point six (=0.4×1.5.) If reference values vary according to wavelengths, alarm control will also be badly affected in this way.
Now, another problem with conventional optical transmission apparatuses will be described. Each vendor has placed importance on the development of more low-priced optical modules of higher quality for optical transmission apparatuses. However, conventional software for calculating performance parameters, such as an LBC, can be used only for a specific optical module. Therefore, software must also be revised to use a newly developed optical module. This is inefficient.